such a modifacation is not possible.the closest thing to a 4 speed changeover would be a saginaw or munse trans. Which wi=ould require a different driveshaft and rearend yours is a closed shaft and the others are open.the 2 trans suggested will bolt up to your bell housing if bearing retainer is same size.a 54 -early 55 rear axle will bolt right in place .if newer axle is used be sure to accomidate for spring center bolt not being centered on axle.